1. Field of the Invention
The present invention relates to a system for optimally controlling the operation of an internal combustion engine by adjusting the net flow rate of intake air sucked into the engine or the charging efficiency of the intake air in an appropriate manner.
2. Description of the Prior Art
FIG. 9 shows a general arrangement of a fuel injection system for an internal combustion engine employing an air-flow sensor (hereinafter referred to as an AFS) adapted to detect the flow rate of intake air sucked into the engine. In FIG. 9, the fuel injection system illustrated comprises an air cleaner 1, a hot-wire type AFS 2, a throttle valve 3 adapted to control the flow rate of intake air sucked into the engine, a surge tank 4, an intake manifold 5, an intake valve 6 adapted to be operated by an engine crank shaft (not shown) through the intermediary of a valve operating mechanism (not shown), a plurality of engine cylinders 7 only one of which is actually illustrated for simplification, a fuel injector 8 provided for each of the engine cylinders 7, and an electronic control unit 9 (hereinafter referred to as an ECU) for controlling the amount of fuel injected from each fuel injector 8 in relation to the flow rate of intake air sucked into the corresponding engine cylinder 7 in such a manner as to provide a predetermined air/fuel ratio. The ECU 9 functions to determine the amount of fuel injected by the respective fuel injectors 8 on the basis of control signals from the AFS 2, a crank-angle sensor 10 for detecting the rotation angle of the engine crank shaft (not shown), a starter switch 11, and a temperature sensor 12 adapted to detect the temperature of engine coolant. Also, the ECU 9 operates to control the pulse width of an electric pulse signal for each of the fuel injectors 8 in synchronization with a signal from the crank-angle sensor 10. In this connection, the crank-angle sensor 10 may be of any known type of sensor which acts to generate rectangular-shaped wave signals which fall at top dead center (hereinafter referred to as TDC) and rise at bottom dead center (hereinafter referred to as BDC) as the engine rotates.
FIG. 10 is a block diagram for explaining, in further detail, the operation of the ECU 9. In this Figure, the ECU 9 includes a revolution-number detecting section 9a for determining the number of revolutions of an engine by measuring a cycle of rectangular-shaped wave signals between adjacent TDCs; an average air-amount detecting section 9b for averaging the output signals from the AFS 2 between adjacent TDCs of the respective rectangular-shaped wave signals fed from the crank-angle sensor 10; a basic pulse-width arithmetic operation section 9c for determining a basic pulse width by dividing an average air flow output from the average air-amount detecting section 9b by a revolution-number output from the revolution-number detecting section 9a; a warming-up revising section 9d adapted to determine a revision coefficient corresponding to the temperature of an engine coolant detected by the temperature sensor 12 for revising the basic pulse width obtained by the basic pulse-width arithmetic operation section 9c by adding or multiplying thereto the revision coefficient so as to provide an optimal injection pulse width; a starting pulse-width arithmetic operation section 9f for determining an appropriate starting pulse width dependent upon the detected temperature of engine coolant; a switch 9g adapted to select the injection pulse width or the starting pulse width in response to an output signal from the starter switch 11 which acts to detect the starting point in time of the engine; and a timer 9h adapted to permit the injection pulse width or the starting pulse width as selected to operate in a one-shot motion at falling points (TDCs) of the output signal of the crank-angle sensor 10 whereby the fuel injectors 8 are driven to operate by means of an injector drive circuit 9i.
As is well known, the basic amount of fuel injected by each of the fuel injectors 8 is in proportion to the flow rate of air sucked into each engine cylinder 7 per revolution of the engine (or charging efficiency of intake air), and a process for determining arithmetic operation for the basic amount of fuel injected by each fuel injector 8 will be described below in detail with reference to FIG. 11.
As illustrated in FIG. 11(a), a crank-angle signal from the crank-angle sensor 10 has falling points corresponding to TDCs and rising points corresponding to BDCs with intervals between adjacent TDCs being at a crank angle of 180.degree.. FIG. 11(b) shows a change in the flow rate of intake air during acceleration of the engine in which a solid line curve A corresponds to the output signal of the AFS 2 and a two-dot long and two short dashes line curve B corresponds to the output signal of the average air-amount detecting section 9b which represents an average of the AFS signal A between adjacent TDCs, and on the basis of which an appropriate amount of fuel to be injected by each fuel injector 8 is calculated. A broken line curve C represents a vacuum signal indicative of a vacuum in the intake manifold 5 which is approximate to the net flow rate of air actually sucked into the respective engine cylinders 7.
From FIG. 11, it will be seen that during transitional operating periods of the engine such as when accelerating, the flow rate of air (curve A) measured by the AFS 2 becomes far greater than the net flow rate of air (curve C) actually sucked into the respective engine cylinders 7. This is because the flow rate of air measured by the AFS 2 involves, in addition to the flow rate of air supplied to the respective engine cylinders 7, the flow rate of air charged into those portions of the intake passage downstream of the throttle valve 3 which include the surge tank 4 and the intake manifold 5. Such a difference becomes particularly remarkable in the case of an intake arrangement layout in which the volume of the engine cylinders 7 is large in comparison with the volume of the surge tank 4.
FIGS. 11(c) through 11(f) show injection pulses when fuel is simultaneously injected into the respective engine cylinders 7 by the respective fuel injectors 8 in a four-cylinder internal combustion engine, in which the solid lines represent pulses based on the net flow rate of air actually sucked into the respective engine cylinders 7, and the broken lines represent pulses based on the flow rate of air clipped by the flow rate of air at the time of the full opening of the throttle valve 3. In this manner, the surplus amounts of the pulse widths, directly calculated by the flow rate of intake air (the curve A) measured by the AFS 2, are suppressed.
With the conventional fuel-injection control of the L-Jetronic type as described above, the flow rate of intake air measured by the AFS 2 and divided by the number of engine revolutions is utilized as the basic fuel-injection amount so that during a transitional operating state of the engine such as engine acceleration, it is difficult to control engine operation in accordance with the net flow rate of air actually sucked into the respective engine cylinders 7.